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Abstract

Choroid plexus epithelial cells (CPECs) secrete cerebrospinal fluid (CSF) and regulate its electrolyte composition. CPECs express Na+/K+ ATPase and Na+-K+-2Cl– cotransporter 1 (NKCC1) on their apical membrane (CSF-facing), deviating from typical basolateral membrane location in secretory epithelia. Given this non-canonical location of NKCC1 for a secretory epithelial cell, the direction of net ion fluxes mediated by this cotransporter and associated water fluxes, under physiological conditions, is controversial in CPECs. NKCC1-mediated ion and water fluxes are tightly linked, thus their direction can be inferred by measuring cell volume changes following NKCC1 inactivation. Hypotheses: under physiological conditions NKCC1 is working in the uptake mode and maintains normal cell water volume in CPECs, hence knocking out NKCC1 will produce cell shrinkage due to unbalanced net efflux of solutes and water. Electron microscopy revealed NKCC1–/– CPECs are severely shrunken, forming large dilations of their basolateral extracellular spaces, yet remain attached by apical tight junctions. Nomarski microscopy confirmed that freshly dissociated CPECs from NKCC1–/– mice are half the size of CPECs from WT (p< 0.01). These results demonstrate that under physiological conditions NKCC1 is constitutively active, works in the uptake mode, and is necessary to maintain normal cell water volume.